1. Field of the Invention
The present invention relates to a thermal printer with a thermal head for line-wise heating a heat-sensitive sheet to produce an image, in particular an image on a transparent support, for medical diagnostic purposes.
2. Description of the Prior Art
Thermal imaging or thermography is a recording process wherein images are generated by the use of image-wise modulated thermal energy.
In thermography two approaches are known:
1. Direct thermal formation of a visible image pattern by the image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 2. Thermal dye transer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element into a receptor element.
A survey of "direct thermal" imaging methods is given in the book "Imaging systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography".
Common thermal printers comprise a rotatable drum and an elongate thermal head which is spring-biased towards the drum to firmly line-wise contact a heat-sensitive material which is passed between the head and the drum.
The thermal head includes a plurality of heating elements and corresponding drivers and shift registers for these elements. The image-wise heating of a sheet is performed on a line by line basis, with the heating resistors geometrically juxtaposed along each other in a bead-like row running parallel to the axis of the drum. Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element.
It is common practice to compensate for across-the-head unevenness. An example of a compensation method has been described in patent application EP-A-0 627 319. According to this method, which is applicable to direct thermal and thermal transfer printing processes, a test print is produced. Density values are measured on this test print and deviations of measured density values from desired values are used to calculate a set of correction values for each heating element.
The test print is produced by first measuring the electrical resistance of each heating element of the printhead and then driving the elements so that the same electrical power as a time average is converted in each of the heating elements.
By the use of the "power-compensated control data" instead of uncorrected control data both in the determination of the density correction factors and during each printing operation, the uniformity of the print results is improved.
In thermal dye transfer the sheet, i.e. the image receiving sheet, is attached to the rotatable drum, and a dye donor sheet or web is conveyed by frictional contact with the rotating sheet past the thermal head.
In direct thermal image formation, a single heat-sensitive sheet is conveyed between the thermal head and the drum, and the image is directly produced in the sheet. The sheet is not attached to the drum but is advanced between the head and the drum by frictional contact of its rear side with the drum.
The present invention is concerned with such direct thermal image formation.
A difficulty with direct thermal image formation is the exact location of the heat-sensitive sheet between the print drum and the thermal head, prior to printing.
In thermal dye transfer printing the image receiving sheet is clamped on the print drum and, prior to printing, the thermal head is lifted from the drum to allow the clamp to pass under the head and the print drum to take the correct angular position in which the leading sheet end extends just beyond the thermal head. In other words, careful control of the angular position of the drum automatically entails the correct start position of the leading sheet end.
In direct thermal image formation, on the contrary, the leading sheet end is freely passed between head and drum before the head becomes urged towards the drum to start printing. The correct position of the leading sheet end preparatory to printing is very important since once the head is closed, the sheet position with respect to the drum can no longer be altered. Although the size of the, unprinted, leading end of the sheet is important in printing positive-type conventional images, it is even more important in printing negative-type images on a transparent support, such as images on a transparent support for medical diagnostic purposes. Examples of medical diagnosis are echograms, CT scans and NMR images which all are negative-type images, what means that the overall background is substantially black, the image details having lesser optical densities. The support of the image-receiving sheet in such application is polyethylene terephthalate or a similar material, the edges of which are often sharp and destructive for any surface in sliding contact therewith. For that reason the width of the sheet to be printed may be smaller than the length of the thermal head but the lateral sheet margins are not printed, and the head is put in contact with the sheet only after the front edge thereof has passed, and removed before the trailing edge of the sheet arrives. Further, the sheet is duly laterally aligned before the thermal head takes its printing position. The result of all this is an unprinted, transparent marginal frame surrounding the image on the sheet which as such is not disturbing because its width is of the order of magnitude of 5 mm only. For reasons of appearance, however, it is important that this margin be of uniform width and thus it is clear that the printing apparatus should provide a very accurate control of the longitudinal and lateral sheet position prior to printing.